Thin film optical shutter



JDU-JOJ H OR 3 9 38 O I 792 7 j y Q April 30, 1968 I E. c. LETTER3,380,792

THIN FILM OPTICAL SHUTTER Filed Jan. 5, 1964 EUGENE C. LETTER INVENTOR.

ATTORNEYS United States Patent" Q ABSTRACT OF THE DISCLOSURE An opticalshutter including a pair of conductive films separated by a dielectricand mounted on a substrate forming a reflective element. A pair ofelectrodes are connected to the conductive films for applying a highvoltage between the two films for destroying the films and the refletivit of the shutter.

This invention relates to an improved optical shutter and moreparticularly to an improved optical shutter or light valve. The presentinvention is an improvement on my copending applications for a HighSpeed Optical Shutter," Ser. No. 544,929, filed Apr. 25, 1966 as acontinuation-in-part of an application Ser. No. 159,105 filed Dec. 13,1961 now abandoned and Improved Optical Shutters," Ser. No. 168,974filed Jan. 26, 1962, now US. Patent No. 3,294,469 which are assigned tothe same assignee as the present application.

Studies of combustion, corona discharge, explosions, plastic and elasticdeformation and shock wave phenomena frequently call for detailedphotographs taken at shutter speeds of a few microseconds. Ballisticsdynamic testing and chemical reactions also require pictures taken atsimilar speeds. At relatively high speeds, it is desirable to shutter arelatively large aperture. The relatively large aperture allows adequatelight to enter the optical shutter during the short time intervals.

A high speed optical shutter according to the present invention may beopened or closed in less than 25 microseconds. In some cases the openingor closing speed 45 approaches one microsecond. A system of this typemay be used in combination with framing cameras. Further, because of therelatively large aperture therein i.e., approximately 1 square, thedevices are particularly applicable in any area requiring a relativelyhigh speed light valve.

Advantageously, a shutter or light valve according to the invention maybe triggered by an event itself. For example, a fiash of light may beused to trigger the valve or electronic means may be incorporated forthat purpose.

Briefly, an optical element according to the present invention includesa mirror comprising a substrate, a pair of electrically conducting thinfilms and a dielectric film separating the electrically conductingfilms. The films are deposited on the substrate and each of theelectrically conducting films is connected to means for producing a highvoltage discharge. A discharge is produced across the pair of films tothereby destroy the reflectivity and change the condition of theshutter. In some cases it is desirable to place the opaque or reflectingsurface i.e.,

the electrically conducting thin film and substrate in an inert gas.Helium has been found to be the most desirable of the inert gases,however, air is also a satisfactory atmosphere. 0

According to a preferred embodiment of the invention, a dielectric filmof approximately A wavelength thick is deposited on the substratebetween the substrate and a first of the electrically conducting films.The wavelength used to indicate the optical thickness is .near themiddle of the spectral region over which improved transmission isdesired. For example, a A wavelength optical thickness selected fromnear the middle of the visible region of the spectrum would act as ananti-reflectant and improve the transmission of visible light.

It is also contemplated to deposit SiO on a high index glass substratein order to obtain advantageous results.

It may also be desirable to deposit a similar dielectric film on top ofthe second electrically conducting film. The second film also tends toimpair the optical characteristics of the device.

The invention will now be described in connection with the accompanyingdrawings; in which,

FIG. 1 is a perspective view showing a shutter according to a firstembodiment of the invention;

FIG. 2 is a schematic illustration of a shutter such as the one shown inFIG. 1 and including means for opening or closing the shutter;

FIG. 3 is a cross-sectional view of the shutter shown in FIG. 1;

FIG. 4 is a perspective view illustrating a shutter according to theinvention disposed in an inert atmosphere; and

FIG. 5 is a schematic illustration showing an optical system includingan improved optical shutter according to the present invention.

The first embodiment of the invention will be described in connectionwith FIGS. 1 through 3 wherein like reference numerals have been used toindicate similar parts. A substrate 2 preferably of glass has arelatively thin dielectric film 3 deposited thereon. A relatively thinelectrically conducting film 4 is deposited on top of the dielectricfilm 3. The film 4 may consist of aluminum, magnesium, an alloycontaining aluminum or magnesium, or some other suitable material suchas silver. An aluminum film is utilized in the presently preferredembodiment of the invention. However, other materials are alsosatisfactory. The thickness of the aluminum film is such that it has anoptical density of approximately 2.0 to 4.0.

A second dielectric film 5 having a thickness of approximately 10/4x isdeposited on top of the film 4. The film 5 separates the electricallyconducting film 4 from a second electrically conducting film 6. Thethickness of this film 6 is such that its optical density isapproximately 2.0 to 4.0. In some cases it is also desirable to deposita third dielectric film 7 such as a layer of silicon dioxideapproximately /2A thick on top of the film 6. The layer of silicondioxide acts as a protective coating and protects the electricallyconducting film 6 from damage by physical abrasion.

An electrically conducting film 4 is connected by a lead 8 to a switch10. The switch 10 is connected to a capacitor 9 by a lead 11. Thecapacitor 9 is also connected to the second electrically conducting film6 by means of a lead 12. Means 14 for producing a high voltage dischargeof between 2,000 and 10,000 volts is connected to the capacitor 9 by apair of electric conducting leads 13, 13'. The two films 4 and 6 areconnected in parallel with the capacitor 9 when the switch 10 is closed.The capacitor 9 may, for example, have a capacitance of between and 2microfarads. The capacitor 9 is charged by the high voltage supply 14and discharges across the pair of films 4 and 6 when the switch 10 isclosed. The electric discharge passing through the pair of filmsdestroys the films 4 and 6 to thereby open or close the shutter.

FIG. 4 illustrates a second embodiment of the invention wherein ashutter assembly such as the one shown in FIG. 1 is disposed in an inertgas. As illustrated in FIG. 4, the assembly is connected by the leads8', 12 to the outside of an envelope 40. The envelope 40 contains aninert gas such as helium. The leads are connected to a capacitor andvoltage supply (not shown) according to the schematic diagram shown inFIG. 2.

In making a shutter according to the invention, it has been founddesirable to evaporate a magnesium fluoride film onto the surface of theshutter prior to depositing the electrical conducting layers. Thesubstrate should be thoroughly cleaned prior to making the deposition inorder to improve the uniformity of the layers. Conventional evaporationtechniques are used to deposit the electn'cally conducting films and thedielectric film separating the electrically conducting films. Similarly,conventional techniques are used to apply a protective coating over thesecond electrically conducting film.

The schematic illustration shown in FIG. shows a shutter 50 according tothe present invention as it might be used in an optical device. Asillustrated, the light rays are reflected into the device by an inclinedmirror 52 through a lens 54 and onto the shutter 50. The reflectivesurface of the shutter 50 directs the light rays incident thereupontoward a lens assembly 55. The lens assembly 55 may be a camera lens orother lens for forming an image at a selected image plane. When thereflectivity of the shutter 50 is destroyed, the light rays are notreflected thereby but pass through the shutter 50 thereby missing theassembly 55.

What is claimed is:

1. An optical shutter comprising:

a reflective element including first and second electrically conductingthin films, a dielectric film separating said electrically conductingthin films and a substrate supporting said films, and

first and second electrodes connected to said first and secondelectrically conducting films respectively for connection across asufiicient high voltage source for current flow therebetweentgjgmjfliifilggs W 2. An optical shutter according to claim in whichsaidelectrically conducting films consist essentially of aluminum. 7

3. An optical shutter according to claim 1 in which the electricallyconducting films have an optical density of about 2 to 4 and saiddielectric film separating said pair of electrically conducting filmsconsist essentially of magnesium fluoride having a thickness of abouttenquarters Wavelengths of light.

4. An optical shutter according to claim 1 in which a thin film ofmagnesium fluoride separates said first electrically conducting filmfrom said substrate, and a layer of silicon dioxide about one-halfwavelength thickness is deposited on said second electrically conductingfilm to thereby protect said second film.

5. An optical shutter according to claim 4 in which the means forproducing a high voltage discharge includes a capacitor having acapacitance of between A and 2 microfarads and electrical meansproducing between 2,000 and 10,000 volts.

References Cited UNITED STATES PATENTS 2,352,085 6/1944 Dimmick 350l642,478,385 8/1949 Gaiser 350-164 2,750,832 6/1956 Morgan 350-1643,294,469 12/1966 Letter 350-463 OTHER REFERENCES Muirhead et al.,Rapid-Opening Electronically-Operated Shutter Rev. Sci. Instruments,vol. 32, N0. 10 (Oct. 1961) pp. 1148 and 1149.

JEWELL H. PEDERSEN, Primary Examiner.

W. L. SIKES, Assistant Examiner.

